AVRO-4026: [C++] Allow non-string custom attributes

[wgtmac]

What is the purpose of the change

This PR takes a different approach than #3308, #3069, #3064 and #3266 to fix the non-string custom attributes. It extends the CustomAttributes::addAttribute(const std::string &name, const std::string &value) with a third parameter bool addQuotes with default value set to true to support adding non-string values explicitly and keep backward compatibility.

Verifying this change

• Make sure that all existing test cases are passed so backward compatibility is preserved.
• Added new test case to verify that both json->schema->json and schema->json->schema round trips have preserved non-string attributes correctly.

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? added comment to explain the behavior

[wgtmac]

This is my attempt to fix this issue after getting more familiar with this codebase. Please take a look, thanks! @thiru-mg @martin-g @Fokko @jhump

(This is the last blocker for me to integrate avro-cpp with iceberg-cpp)

[wgtmac]

I can also mark it as deprecated and add two clearer functions like:

[[deprecated("Deprecated in 1.13.0 and will be removed in 1.14.0. Use addStringAttribute instead.")]]
void addAttribute(const std::string &name, const std::string &value, bool addQuotes = true);

void addStringAttribute(const std::string &name, const std::string &value) {
    addAttribute(name, value, /*addQuotes=*/true);
}

void addNonStringAttribute(const std::string &name, const std::string &value) {
    addAttribute(name, value, /*addQuotes=*/false);
}